kmplayer is only a front-end/GUI for already installed player like gstreamer and mplayer. BUT: for gstreamer the mediaplayer-gui is the best. for mplayer i would say that SIB is the smartest GUI - it's not as shiny as kmplayer but it's way less resource hungry and it seems that it starts mplayer with the best options to optimize it for use on the n900.
mplayer + SIB simply kicks ass. forget gstreamer and vlc for now. ;)

A bit slow.
Haven't been able to play any local videos though; screen starts flashing uncontrollably, and I've only been able to hard-reboot to fix it.
Not all terrible, but could certainly use some work. Maybe demote to extras-testing and go through QA again?